1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a facsimile machine, a printer, and a multi function mechanism. The invention also relates to a sheet processing apparatus which performs processing such as alignment, binding, and center-folding to image-formed sheets which are discharged from the image forming apparatus. The invention also relates to a sheet processing method.
2. Description of the Related Art
The image forming apparatus such as the copying machine is provided with a sheet post-processing apparatus which stacks plural image-formed sheets to perform saddle stitch or the like. The sheet bundle which is center-folded at a binding position is discharged and stacked on a discharge tray with a folded portion in the lead. When the folded sheet bundle has a thick, or when the sheet has strong stiffness, it is difficult that the sheet bundle is completely folded, and sometimes the sheet bundle becomes poor-looking because both ends are opened after the sheet bundle is folded. When the sheet bundle is loosely folded to easily open both the ends, the subsequent sheet bundle plunges into the already stacked sheet bundle in stacking the plural sheet bundles on top of another on a discharge tray. When the sheet bundles are stacked on top of another while swollen, the sheet bundles easily collapse.
In order to eliminate the defect, for example, Japanese Patent Application Laid-Open No. 11-193175 discloses a sheet bundle folding apparatus and a sheet processing apparatus. In this case, when the folded portion of the sheet bundle is delivered toward a nip between a pair of folding rollers, a pushing member plunges the folded portion of the sheet bundle between the folding rollers while being followed by motion of the sheet bundle. Therefore, the sheet bundle is completely folded by correctly and securely pushing the folded portion of the sheet bundle with the pushing member.
However, in the sheet bundle folding apparatus and sheet processing apparatus disclosed in Japanese Patent Application Laid-Open No. 11-193175, there are problems to be solved.
One of the problems is that, in the case where the folding processing is performed to the sheet bundle, the fold becomes looser in the sheet located close to the outside of the folded sheet bundle. This is attributed to the fact that a folded radius becomes larger in the sheet located close to the outside due to a thickness of the sheet bundle.
Another problem is that, during an action in a staple mode selected by a user, sometimes breakdown or wrinkle is generated in a wire-stitched portion when the sheet bundle is center-folded at the wire-stitched portion with the sheet bundle folding apparatus disclosed in Japanese Patent Application Laid-Open No. 11-193175 after stapling process is previously performed at the predetermined position where the sheet bundle is center-folded. This is because a cover sheet is broken from the wire-stitched portion to generate the wrinkle by a difference between frictional resistance which the cover sheet receives from a roller surface and frictional resistance generated by rubbing the sheets with one another inside the sheet bundle when the sheet bundle is conveyed while nipped between the rollers.
Still another problem is a defect which is generated by bringing the pushing member close to the folding roller to prevent the breakdown and wrinkle as much as possible. In the configuration disclosed in Japanese Patent Application Laid-Open No. 11-193175, when the sheet bundle is nipped between the folding rollers, the pushing member which is stopped just before the sheet bundle rubs the inside of the sheet bundle to generate a scratch and a trace.
In view of the foregoing, an object of the invention is to provide a sheet processing apparatus which obtains the folded sheet bundle having the good appearance and excellent stacking property by performing the folding processing to the sheet bundle twice in an opposite direction, and an image forming apparatus provided with the sheet processing apparatus.